WIPO patent WO2013132009 (A1) published Sep. 12, 2013 in the name of Borealis AG discloses a process and plant for manufacturing polyethylene or ethylene based copolymers. The patent discloses a known set-up for a high pressure polyethylene reactor consisting of two compressors, a polymerization reactor, and two separators (for separating the monomer-polymer mixture leaving the reactor) having recycle feeds that contain dewaxing units. The patent does not teach how the de-waxing units are operated to reduce low molecular weight oligomers and wax build-up at various points in the reactor.
The online Kirk-Othmer Encyclopedia of Chemical Technology Vol. 00 Polyethylene, Low Density at page 38 discloses that the recycle system used in the high pressure polymerization of polyethylene generally comprises two sections: a high-pressure separator and a low-pressure separator. Kirk-Othmer discloses that the high-pressure separator first divides the polymer from the ethylene gases and low molecular weight waxes and that the gases and waxes are then separated and cooled. However, Kirk-Othmer does not suggest a process for reducing low molecular weight oligomers and wax build-up at various points in the reactor.
The present invention seeks to provide a method for reducing low molecular weight oligomers and wax build-up at various points between the high pressure separator and the hypercompressor by separately draining each of n knock-out pots downstream of the high-pressure separator at a frequency of once every 24/n hours, or at least once every 8 hours, preferably at least once every 6 hours. It also seeks to provide a process for increasing the capacity of a high pressure reactor having a fixed reactor volume by increasing the operating pressure of the reactor and draining each of n knock-out pots, where n is greater than 1, preferably greater than 2, downstream of the high-pressure separator at a frequency of once every 24/n hours, or at least once every 8 hours, preferably at least once every 6 hours.